In a power electronic system, a DC bus capacitor is needed to be connected across the DC bus of the power electronic system to store energy. Usually, an electrolytic capacitor with large capacity and high voltage endurance is used as a DC bus capacitor, to store a large amount of energy. However, the electrolytic capacitor is subject to aging or even failure, thus causing system performance degradation or breakdown.
For example, FIG. 1 is a schematic diagram of a power electronic converter module 1000 in a phase of a power electronic transformer (PET) in the prior art. The power electronic converter module 1000 includes an AC (Alternative Current)/DC conversion circuit (AC/DC) 1 and a DC/DC conversion circuit (DC/DC) 2. The AC/DC 1 and DC/DC 2 are cascaded through a DC bus B. A DC bus capacitor C is connected in parallel with the DC bus B. The power electronic converter module 1000 has an input terminal, i.e., an input terminal Tin of the AC/DC 1, and an output terminal, i.e., an output terminal Tout of the DC/DC 2. The input terminal Tin, may be connected to an AC power source, such as an AC power grid, a wind turbine, etc., to input an AC voltage Vi with a frequency f, wherein f is any positive real number. The output terminal Tout may output a DC voltage Vo to a load. During operation of the power electronic converter module 1000, since both the capacity of the AC/DC 1 and the capacitance of the DC bus capacitor C are limited, the voltage on the DC bus capacitor C is typically a pulsating DC voltage, where the frequency of the main pulsating component is 2f (i.e., two times of f), which is particularly disadvantageous to the life expectancy of the electrolytic capacitor.
Therefore, in order to avoid decline of reliability of a power electronic system due to failure and aging of the electrolytic capacitor, the capacitance of the DC bus capacitor needs to be monitored to predict which DC bus capacitor is about to fail, so as to take appropriate measures in advance.
In the authorization bulletin No. CN103580497B, the capacitance prediction value of the DC bus capacitor is corrected by a closed loop, and the corrected capacitance is substituted into a capacitance voltage dynamic equation to predict the current voltage of the DC bus capacitor. When a voltage prediction value and a measured value of the DC bus capacitor are the same, the corrected value of the capacitance at this time is considered to be the same as the actual value. In such a way to predict the capacitance of the DC bus capacitor, difference between the voltage prediction value and the measured value of the DC bus capacitor will simultaneously affect the input power prediction value and the capacitance prediction of the DC bus capacitor, which may affect the accuracy of capacitance estimation.
In the authorization bulletin CN103795284A, a single module of inverter to be measured in cascaded inverters is selected through a selection signal. The phase of its output voltage is modified to have a phase difference of 90° with its load current. Output voltages of remaining inverters of the same phase are adjusted at the same time, such that the total output voltage of the phases remains unchanged. The power of the DC bus capacitor of the module to be measured only has an AC component. The capacitance of the DC bus capacitor of the module to be measured may be estimated by using a quantitative relationship between peak to peak value of the fluctuating voltage of the DC bus capacitor and an AC power component of the second-order harmonic frequency. However, this way needs to change the normal operation mode, and a reference voltage of the module to be measured needs to be set separately.